Geochemical studies have been carried out on the hanging wall argillites to better constrain the role of sedimentary processes in ore formation. A few samples have S/C, degree-of-pyritization, and base- and precious metal concentrations like normal marine shales, but most are higher in these parameters. Dolomitic argillite has δ18O and δ13C values of 19±3 and 6±2, respectively, lower than normal marine carbonate but indistinguishable from the δ-values of hydrothermal dolomite within the orebody. Organic carbon, which is present in argillite in varying amounts, has a δ13C value of 25.9±0.9; the carbonate-organic carbon fractionation is uniform across a wide range of organic carbon contents. Dolomite δ13C is weakly correlated with organic carbon δ13C and with sulfur content.
We infer that (1) the argillite section experienced epigenetic additions of sulfur and metals, (2) the fluid that deposited sulfide-stage dolomite in the orebody also affected the overlying argillite section, (3) dolomite precipitated either from a single fluid over a range of temperatures or from a mixture of marine and hydrothermal fluids, and (4) H2S was produced from marine sulfate by oxidation of sedimentary organic matter or organic-derived methane.